Originally Posted by briandg
In the case of the ar, iirc, the firearm is the upper receiver assembly, not the lower with trigger group and fire control.
I DON'T KNOW WHY.
You have that part wrong, no offense. On an AR-15 type firearm, the lower receiver is the serial numbered part.
Perhaps you have it confused with a FAL type rifle, in which the upper receiver is the serial numbered part.